I'm with the rest of the crowd, probably Rocephin or Bicillin.
I will often start a line just to give Rocephin instead of giving it IM. However, when I DO give it intramuscularly, I will mix it with Lidocaine, give it z-track, and rapidly pat the area around the injection site while I'm pushing it slowly. It confuses the nerve endings, I learned that from a dentist.
Bicillin hurts like the dickens, but I've always been so sick from strep throat that I really didn't care whether it did or not. I'd rather withstand a little pain and get better sooner.
Phenergan has a pH of 4, so it BURNS, too. The trick is to give it slowly IM, or to dilute the heck out of it IV. I give MYSELF phenergan shots whenever I'm vomiting, so again, I usually am so sick I don't care.
I've heard people say that Depo hurts, but I've given it to myself before and didn't notice.